1. Field of the Invention
The present invention relates to a composite material and a method for producing the same, the composite material being used for heat sinks for semiconductor devices, for constructing a heat sink for a semiconductor device for efficiently releasing heat generated from the semiconductor device.
2. Description of the Related Art
In general, heat is a dangerous enemy of semiconductor devices. Therefore, it is necessary that the internal temperature of the semiconductor device does not exceed a maximum allowable temperature for retaining the joining or connecting structure. Semiconductor devices such as power transistors and semiconductor rectifying elements consume a large amount of electric power per unit of operation area. Therefore, it is impossible to release a sufficient amount of the generated heat only by relying on an amount of heat released through a case (package) and lead wires of the semiconductor device. In such a circumstance, there is a fear that the internal temperature of the device is raised, and any thermal destruction would occur.
This phenomenon also occurs in the same manner in semiconductor devices which carry a CPU. The amount of heat generation during operation is increased in proportion to the improvement in clock frequency. As a result, it is an important factor to make a thermal design in consideration of heat release.
In the thermal design in consideration of avoidance of the thermal destruction or the like, element designs and mounting designs are made taking account of a heat sink having a large heat release area which is securely attached to a case (package) of the semiconductor device.
In general, those used as the material for the heat sink include metal materials such as copper and aluminum having good thermal conductivity.
Recently, in semiconductor devices such as CPUs and memories, it is intended to drive the device with low electric power in order to decrease electric power consumption, while the semiconductor device itself tends to have a large size in proportion to highly densified element integration and enlargement of element formation area. When the semiconductor device has a large size, the stress, which is generated due to the difference in thermal expansion between the semiconductor substrate (silicon substrate or GaAs substrate) and the heat sink, is increased. As a result, there is a likelihood of delamination, and and mechanical destruction of the semiconductor device.
In order to avoid such inconveniences, the conceivable countermeasure includes realization of low electric power operation of the semiconductor device and improvement of the material for heat sinks. At present, in relation to the low electric power operation of the semiconductor device, a device, which is operated at a power source voltage of a level of not more than 3.3 V, is practically used, beyond those operated at the TTL level (5 V) having been hitherto used.
On the other hand, in relation to the constitutive material for the heat sink, it is insufficient to consider only the thermal conductivity. Besides, it is necessary to select a material having high thermal conductivity with a coefficient of thermal expansion which is approximately coincident with those of silicon and GaAs to be used for the semiconductor substrate.
A variety of reports have been submitted in relation to the improvement in material for the heat sink. For example, there is a case based on the use of aluminum nitride (AlN) and a case based on the use of Cu (copper)-W (tungsten). AlN is excellent in balance between the thermal conductivity and the thermal expansion, and it especially has a coefficient of thermal expansion which is approximately coincident with that of Si. Therefore, AlN is preferred as a material for heat sinks for the semiconductor device based on the use of a silicon substrate as the semiconductor substrate.
On the other hand, Cuxe2x80x94W is a composite material which possesses both the low thermal expansion of W and the high heat conductivity of Cu, and it easily processed by means of machining. Therefore, Cuxe2x80x94W is preferred as a constitutive material for heat sinks having complicated shapes.
There are other suggested cases including, for example, a material obtained by containing metallic Cu in a ratio of 20 to 40% by volume in a ceramic base material comprising a major component of SiC (Conventional Example 1: see Japanese Laid-Open Patent Publication No. 8-279569), and a material obtained by impregnating a powdery sintered porous body comprising inorganic substances with Cu in an amount of 5 to 30% by weight (Conventional Example 2: see Japanese Laid-Open Patent Publication No. 59-228742).
The material for heat sinks concerning Conventional Example 1 is based on powder shaping in which a green compact comprising SiC and metallic Cu is shaped to prepare a heat sink. Therefore, the coefficient of thermal expansion and the coefficient of thermal conductivity thereof are persistently represented by theoretical values. In this case, there is a problem that it is impossible to obtain the balance between the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like.
In Conventional Example 2, the ratio of Cu, with which the powdery sintered porous body comprising inorganic substances is impregnated, is low. Therefore, there is a fear that a limit appears when it is intended to increase the thermal conductivity.
The present invention has been made taking such problems into consideration, an object of which is to provide a composite material for heat sinks for semiconductor devices, which makes it possible to obtain characteristics adapted to balance the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like (including semiconductor devices).
Another object of the present invention is to provide a method for producing a composite material for heat sinks for semiconductor devices, which makes it possible to easily perform a treatment for impregnating a porous sintered compact with a metal although such a treatment is generally considered to be difficult, making it possible to improve the rate of impregnation of the metal into the porous sintered compact, and making it possible to improve the productivity of the heat sink which has characteristics adapted to balance the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like (including semiconductor devices).
At first, explanation will be made for the optimum characteristics as the material for heat sinks. The required coefficient of thermal expansion is preferably in a range of 4.0xc3x9710xe2x88x926/xc2x0 C. to 9.0xc3x9710xe2x88x926/xc2x0 C. as an average coefficient of thermal expansion from room temperature to 200xc2x0 C., because it is necessary to conform to the coefficient of thermal expansion of the ceramic substrate such as those composed of AlN and the semiconductor substrate such as those composed Si and GaAs. The required coefficient of thermal conductivity is preferably not less than 180 W/mK (room temperature), because it is necessary to satisfy the requirement equivalent or superior to those satisfied by the presently used Cuxe2x80x94W material.
According to the present invention, there is provided a composite material for heat sinks for semiconductor devices, comprising a porous sintered compact impregnated with copper or a copper alloy, the porous sintered compact being obtained by pre-calcinating a porous body having a coefficient of thermal expansion which is lower than a coefficient of thermal expansion of copper so that a network structure is formed; wherein the composite material has a characteristic that at least a coefficient of thermal expansion at 200xc2x0 C. is lower than a coefficient of thermal expansion which is stoichiometrically obtained on the basis of a ratio between the copper or the copper alloy and the porous sintered compact.
According to the present invention, it is possible to suppress the expansion to be at a value which is lower than the thermal expansion (theoretical value) determined by the ratio between the porous sintered compact and the copper or the copper alloy with which the porous sintered compact is impregnated. The coefficient of thermal expansion is approximately coincident with those of, for example, ceramic substrates and semiconductor substrates (silicon, GaAs). Thus, it is possible to obtain a material for heat sinks having good thermal conductivity.
Specifically, it is possible to obtain a material for heat sinks in which an average coefficient of thermal expansion in a range from room temperature to 200xc2x0 C. is 4.0xc3x9710xe2x88x926/xc2x0 C. to 9.0xc3x9710xe2x88x926/xc2x0 C., and a coefficient of thermal conductivity is not less than 180 W/mK (room temperature).
It is desirable that the porous sintered compact comprises at least one or more compounds selected from the group consisting of SiC, AlN, Si3N4, B4C, and BeO. It is desirable that the ratio (impregnation rate) of the copper or the copper alloy is 20% by volume to 70% by volume. If the rate of impregnation of copper is less than 20% by volume, it is impossible to obtain the coefficient of thermal expansion of 180 W/mK (room temperature), while if the rate of impregnation exceeds 70% by volume, then the strength of the porous sintered compact (especially SiC) is lowered, and it is impossible to suppress the coefficient of thermal expansion to be less than 9.0xc3x9710xe2x88x926/xc2x0 C.
It is desirable that a value of an average open pore diameter of the porous sintered compact is 0.5 to 50 xcexcm. If the value of the average open pore diameter is less than 0.5 xcexcm, then it is difficult to impregnate the open pores with the metal, and the coefficient of thermal conductivity is lowered. On the other hand, if the value of the average open pore diameter exceeds 50 xcexcm, then the strength of the porous sintered compact is lowered, and it is impossible to suppress the coefficient of thermal expansion to be low.
It is preferable that the distribution (pore distribution) in relation to the average open pore of the porous sintered compact is distributed by not less than 90% in a range of 0.5 to 50 xcexcm. If the pores of 0.5 to 50 xcexcm are not distributed by not less than 90%, the open pores, which are not impregnated with copper, are increased. Consequently, the coefficient of thermal conductivity is decreased, or the strength is decreased, and it is impossible to suppress the coefficient of thermal expansion to be low.
It is desirable that bending strength of the porous sintered compact is not less than 10 MPa. If the strength is less than this value, then it is impossible to suppress the coefficient of thermal expansion to be low, and it is impossible to obtain the composite material having the coefficient of thermal expansion in the predetermined range.
In general, when commercially available pure copper is used as the copper, a good composite material having a high coefficient of thermal conductivity is obtained. However, the obtained composite material is not excellent in wettability with respect to the porous sintered compact (especially SiC), and the open pores which are not impregnated with copper tend to remain. Therefore, it is desirable to improve the impregnation rate by adding, for example, Be, Al, Si, Mg, Ti, and Ni. However, if the additive is added in an amount of not less than 1%, then the coefficient of thermal conductivity is greatly decreased, and it is impossible to obtain the effect which would be otherwise obtained by the addition.
It is desirable that a reaction layer, which is formed at an interface between the porous sintered compact and the copper (only copper or one containing copper and Be, Al, Si, Mg, Ti, Ni or the like in a range up to 1%), is not more than 5 xcexcm. More preferably, the reaction layer is not more than 1 xcexcm. If the reaction layer is thicker than 5 xcexcm, then the heat transfer between the porous sintered compact and the copper is deteriorated, and the thermal conduction of the composite material for heat sinks for semiconductor devices is decreased.
In another aspect, the present invention provides a method for producing a composite material for heat sinks for semiconductor devices, the method comprising an impregnating step of heating a porous sintered compact to serve as a base material and a metal containing at least copper, in a state of making no contact with each other, and making contact of the both at a stage of arrival at a predetermined temperature to immediately apply a high pressure so that the porous sintered compact is impregnated with the metal; and a cooling step of cooling the porous sintered compact impregnated with at least the metal.
For example, the porous sintered compact to serve as the base material and the copper or the copper alloy used for impregnation thereof are heated while making no contact with each other. At the stage at which both arrive at a temperature not less than a melting point of the copper or the copper alloy, both are allowed to make contact with each other to immediately apply the high pressure so that the porous sintered compact is impregnated with the copper or the copper alloy, followed by quick cooling.
Accordingly, the treatment for impregnating the porous sintered compact with the copper or the copper alloy, which is generally considered to be difficult, can be performed with ease. Moreover, it is possible to improve the rate of impregnation of the copper or the copper alloy into the porous sintered compact. As a result, it is possible to improve the productivity of the heat sink which has characteristics adapted to balance the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like (including semiconductor devices).
The characteristics adapted to balance the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like (including semiconductor devices) are represented such that the average coefficient of thermal expansion from room temperature to 200xc2x0 C. is 4.0xc3x9710xe2x88x926/xc2x0 C. to 9.0xc3x9710xe2x88x926/xc2x0 C., and the coefficient of thermal conductivity is not less than 180 W/mK (room temperature).
In a preferred embodiment, the impregnating step may comprise the steps of placing the porous sintered compact and the metal into an identical vessel, arranging the metal at a lower portion of the vessel, and then allowing the vessel to be in a negative pressure state or in an ordinary pressure state therein; heating and melting the metal to convert the metal into molten metal; inverting the vessel at a stage at which the molten metal arrives at a predetermined temperature to immerse the porous sintered compact in the molten metal in the vessel; and impregnating the porous sintered compact with the molten metal by introducing an impregnating gas into the vessel to apply a pressure in the vessel.
That is, the porous sintered compact and the copper or the copper alloy to be used for impregnating the porous sintered compact therewith are placed in the vessel, and the vessel is tightly sealed to perform vacuum suction, followed by heating while placing the copper or the copper alloy at the lower portion of the vessel. The vessel is inverted by 180 degrees to be upside down at the stage at which the copper or the copper alloy is melted to arrive at the predetermined temperature. Accordingly, the copper or the copper alloy is allowed to make contact with the porous sintered compact. A high pressure is applied in the vessel so that the porous sintered compact is impregnated with the copper or the copper alloy.
In another preferred embodiment, the impregnating step may comprise the steps of placing the metal having been previously melted and the porous sintered compact into an identical vessel, arranging the molten metal at a lower portion of the vessel, and then allowing the vessel to be in a negative pressure state or in an ordinary pressure state therein; inverting the vessel at a stage at which the molten metal arrives at a predetermined temperature to immerse the porous sintered compact in the molten metal in the vessel; and impregnating the porous sintered compact with the molten metal by introducing an impregnating gas into the vessel to apply a pressure in the vessel.
That is, the previously melted copper or the copper alloy is placed into the vessel in which the porous sintered compact is installed, and the vessel is inverted by 180 degrees to be upside down at the stage at which the contents in the vessel arrive at the predetermined temperature. Accordingly, the copper or the copper alloy is allowed to make contact with the porous sintered compact. A high pressure is applied in the vessel so that the porous sintered compact is impregnated with the copper or the copper alloy.
The applied pressure is not less than 10 kgf/cm2 and not more than 1000 kgf/cm2, preferably not less than 50 kgf/cm2 and not more than 200 kgf/cm2, and more preferably not less than 100 kgf/cm2 and not more than 150 kgf/cm2.
In this embodiment, the pressure is applied for a period of time of not less than 1 minute and not more than 30 minutes, and desirably not less than 2 minutes and not more than 10 minutes.
The predetermined temperature is a temperature which is higher than a melting point of the copper or the copper alloy for impregnation therewith by 30xc2x0 C. to 250xc2x0 C., and preferably the predetermined temperature is a temperature which is higher than the melting point by 50xc2x0 C. to 200xc2x0 C. In this embodiment, it is preferable that the copper or the copper alloy for impregnating the porous sintered compact therewith is heated in vacuum of not more than 1xc3x9710xe2x88x923 Torr.
It is desirable that the porous sintered compact includes pores not less than 90% of which have an average diameter of 0.5 xcexcm to 50 xcexcm, having a porosity of 20% by volume to 70% by volume.
In a preferred embodiment, the porous sintered compact is previously plated with Ni in an amount of 1 to 10% by volume. In this embodiment, the wettability between the porous sintered compact and the copper or the copper alloy is improved, and it is possible to realize impregnation at a low pressure. The amount of the Ni plating is desirably 3 to 5% by volume. The Ni plating referred to herein includes, for example, Nixe2x80x94P plating and Nixe2x80x94B plating.
It is also preferable that the porous sintered compact is previously impregnated with 1 to 10% by volume of Si. In this embodiment, the wettability between the porous sintered compact and the copper or the copper alloy is improved in the same manner as the case of the application of Ni plating described above, and it is possible to realize impregnation at a low pressure. The amount of the impregnation of Si is desirably 3 to 5% by volume.
In relation to the Ni plating previously applied by 1 to 10% by volume to the porous sintered compact, or the previous impregnation of Si by 1 to 10% by volume, it is also preferable that the porous sintered compact is previously plated with palladium. In this embodiment, in addition to the palladium plating, it is also possible to apply composite plating together with Ni and Si.
In still another preferred embodiment, the cooling step may comprise the steps of inverting the vessel to separate the porous sintered compact after the impregnation from the remaining molten metal not subjected to the impregnation; and venting the impregnating gas from the vessel to quickly introduce a cooling gas so that the inside of the vessel is cooled. Alternatively, the cooling step may comprise the steps of inverting the vessel to separate the porous sintered compact after the impregnation from the remaining molten metal not subjected to the impregnation; and allowing the vessel to make contact with a chill block so that the inside of the vessel is cooled.
The cooling step is preferably performed at a cooling rate of not less than xe2x88x92400xc2x0 C./hour from the temperature during the impregnation to 800xc2x0 C., and more preferably not less than xe2x88x92800xc2x0 C./hour.
The applied pressure is a pressure necessary to completely impregnate the open pores of the porous sintered compact with the copper or the copper alloy. In this process, if the open pores, which are not impregnated with the copper or the copper alloy, remain in the porous sintered compact, the heat conductivity is markedly inhibited. Therefore, it is necessary to apply a high pressure.
The pressure can be approximately estimated in accordance with the expression of Washburn. However, the smaller the pore diameter is, the larger the required force is. For example, the required pressure is 400 kgf/cm2 in the case of 0.1 xcexcmxcfx86, 40 kgf/cm2 in the case of 1.0 xcexcmxcfx86, and 4 kgf/cm2 in the case of 10 xcexcmxcfx86 respectively.
A reaction occurs between the porous sintered compact and the copper or the copper alloy in the molten state. For example, when SiC is used as the porous sintered compact, SiC is decomposed into Si and C, and the original function is not exhibited. For this reason, it is necessary to shorten the period of time during which SiC makes direct contact with Cu in the molten state. According to the production method concerning the present, it is possible to shorten the contact time between SiC and Cu. Accordingly, it is possible to avoid the decomposition reaction of SiC as described above beforehand.
The wettability is poor between SiC and the copper or the copper alloy. Therefore, it is necessary to apply the high pressure in order to sufficiently perform the impregnation with the copper or the copper alloy. According to the production method concerning the present invention, the pore surface of SiC is modified in quality to give good wettability between SiC and Cu. Accordingly, it is possible to impregnate finer pores with the copper or the copper alloy at a lower pressure.
In still another preferred embodiment, the impregnating step may comprise the steps of placing the porous sintered compact and the metal in a negative pressure state or in an ordinary pressure state while making no contact with each other; heating the porous sintered compact and the metal to the predetermined temperature at the negative pressure or at the ordinary pressure to melt the metal; allowing the molten metal to be in a pressure-applied state; and allowing the molten metal at the applied pressure to quickly make contact with the porous sintered compact at the negative pressure or at the ordinary pressure and allowing them to be in a pressure-applied state so that the porous sintered compact is impregnated with the molten metal at the applied pressure; and the cooling step may comprise the step of cooling the porous sintered compact impregnated with the molten metal at the applied pressure.
In this embodiment, the porous sintered compact and the metal are heated while performing sufficient deaeration to melt the metal, followed by making quick contact and giving the pressure-applied state. Further, the pressure-applied state is maintained until completion of the cooling operation. Thus, it is possible to efficiently impregnate the porous sintered compact with the molten metal.
In the production method as described above, it is preferable that both of the porous sintered compact and the molten metal, which are heated and treated while making no contact with each other at the negative pressure or at the ordinary pressure, are placed in the pressure-applied state, and then they are allowed to quickly make contact with each other so that the porous sintered compact is impregnated with the metal.
Accordingly, the porous sintered compact is allowed to be in the pressure-applied state together with the molten metal, followed by performing the contact and impregnating operations. Thus, it is possible to minimize the pressure drop which would be caused upon the contact of the both, and it is possible to well maintain the pressure-applied state during the impregnating operation.
In still another preferred embodiment, the impregnating step may comprise the steps of arranging the porous sintered compact and the metal respectively in upper and lower chambers of an identical vessel comparted to have the two chambers by a porous filter, and tightly sealing the vessel so that the respective chambers are in a negative pressure state or in an ordinary pressure state; heating both of the upper and lower chambers at the negative pressure or at the ordinary pressure to a predetermined temperature so that the metal is melted; allowing only the upper chamber to be in a pressure-applied state; and allowing the molten metal in the upper chamber at the applied pressure to permeate through the porous filter to the lower chamber so that the molten metal quickly makes contact with the porous sintered compact at the negative pressure or at the ordinary pressure, followed by allowing the lower chamber to be in a pressure-applied state so that the porous sintered compact at the applied pressure is impregnated with the molten metal; and the cooling step may comprise the step of cooling the porous sintered compact impregnated with the molten metal in the lower chamber in the pressure-applied state.
In this embodiment, the upper chamber arranged with the metal and the lower chamber arranged with the porous sintered compact can be independently subjected to pressure control by using the porous filter. Accordingly, it is possible to quickly reduce or apply the pressure by using a predetermined pressure control mechanism.
The porous sintered compact in the lower chamber can be deaerated while maintaining it in the negative pressure state or in the ordinary pressure state immediately before the impregnation of the molten metal. Further, the contact and impregnating operations for the molten metal and the porous sintered compact can be easily performed in accordance with the pressure control effected by the aid of the porous filter. In this process, the molten metal can be quickly treated with the filter, because of the difference in pressure which is provided beforehand between the both chambers.
The material for the porous filter is not specifically limited provided that the material has a porous property of a degree so that the molten metal makes no permeation at the ordinary pressure and the molten metal makes permeation at the applied pressure. Those preferably usable as the material include, for example, carbon cloth, punching metal composed of stainless steel, and alumina cloth.
In still another preferred embodiment, the impregnating step may comprise the steps of arranging the porous sintered compact and the metal respectively in upper and lower chambers of an identical vessel comparted to have the two chambers by a porous filter, and tightly sealing the vessel so that the respective chambers are in a negative pressure state or in an ordinary pressure state; heating both of the upper and lower chambers at the negative pressure or at the ordinary pressure to a predetermined temperature so that the metal is melted; allowing both of the upper and lower chambers to be in a pressure-applied state; and raising a pressure of the pressure-applied upper chamber to be higher than a pressure of the lower chamber, and allowing the molten metal to permeate through the porous filter to the lower chamber so that the molten metal quickly makes contact with the porous sintered compact, and then the porous sintered compact at the applied pressure is impregnated with the molten metal; and the cooling step may comprise the step of cooling the porous sintered compact impregnated with the molten metal in the lower chamber at the applied pressure.
In this embodiment, the porous sintered compact is allowed to be in the pressure-applied state together with the molten metal, followed by performing the contact and impregnating operations. Thus, it is possible to minimize the pressure drop which would be caused upon the contact of the both, and it is possible to well maintain the pressure-applied state during the impregnating operation.
In the present invention, when the porous sintered compact to serve as the base material is treated and impregnated with the metal containing at least copper, the step of providing the pressure-applied state may be performed by means of a press treatment effected in both upward and downward directions, and the cooling step may be performed by means of an indirect cooling treatment effected in the vicinity of the lower chamber.
According to the production method concerning the present invention, it is possible to more quickly perform the pressure control, and it is possible to well maintain the pressure-applied state during the impregnating operation.
In still another aspect, the present invention provides a method for producing a composite material for heat sinks for semiconductor devices, the method comprising an impregnating step of allowing a porous sintered compact to serve as a base material to make contact with a metal containing at least copper at a negative pressure or at an ordinary pressure, performing a heating treatment to melt the metal, and then quickly impregnating the porous sintered compact with the metal in a pressure-applied state; and a cooling step of cooling at least the porous sintered compact impregnated with the metal.
Accordingly, the treatment for impregnating the porous sintered compact with the copper or the copper alloy, which is generally considered to be difficult, can be performed with ease. Moreover, it is possible to improve the rate of impregnation of the copper or the copper alloy into the porous sintered compact. As a result, it is possible to improve the productivity of the heat sink which has characteristics adapted to the balance between the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like (including semiconductor devices).
In a preferred embodiment, the impregnating step may comprise the steps of placing the porous sintered compact and the metal in a negative pressure state or in an ordinary pressure state while making contact with each other; heating the porous sintered compact and the metal to a predetermined temperature at the negative pressure or at the ordinary pressure to melt the metal; allowing the molten metal to be in a pressure-applied state; and allowing the molten metal at the applied pressure to quickly make contact with the porous sintered compact at the negative pressure or at the ordinary pressure and allowing them to be in a pressure-applied state so that the porous sintered compact is impregnated with the molten metal at the applied pressure; and the cooling step may comprise the step of cooling the porous sintered compact impregnated with the molten metal at the applied pressure.
As described above, according to the production method concerning the present invention, it is possible to suppress the expansion to be at a value which is lower than the thermal expansion amount (theoretical value) determined by the ratio between the porous sintered compact and the copper or the copper alloy with which the porous sintered compact is impregnated. The coefficient of thermal expansion is approximately coincident with those of, for example, the ceramic substrate and the semiconductor substrate (silicon, GaAs). Thus, it is possible to obtain the material for heat sinks having good thermal conductivity.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.